In the 1660's Isaac Newton discovered that a prism, traditionally made of clear glass, could split light into a spectrum of colours when struck by a narrow beam of sunlight. Although translucency is generally though to be the 'natural' appearance of glass, this is not actually true. Clear glass is more difficult to achieve than many coloured glasses became sand, which makes up the bulk of the glassmaking raw material, always contains impurities- usually in form of iron oxide, which lends a greenish/brownish tint. To counteract this, other oxides, for example manganese or cerium oxide, are added as decolourisers. One of the major goals of glass-makers from the fifteenth century onward has been to achieve a perfectly clear glass: a difficult task.
Glass artists extensively play with the optical properties of glass in their work. American artist John Kuhn, for instance, creates glass objects that make use of mirror effects, refraction and reflection. They are cut, polished and assembles from many precisely cut pieces of mostly clear glass, revealing an infinity of shapes and colours. The optical properties of the glass, together with light, result in constant movement and change.
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